Implantable pacemakers have achieved some success in the treatment of tachycardias. Since the primary function of most pacemakers is to maintain the heart rate at or above a minimum value, different circuitry or different modes of operation are usually required for the treatment of tachycardias. Generally speaking, two modes of operation have been proposed: the use of bursts of high frequency pulses, and the use of asynchronous, or fixed rate, pacing.
One way in which re-entry tachycardias may commence is when an atrial premature impulse conducted normally to the ventricle is then able to re-enter the supraventricular region by retrograde conduction via an accessory pathway and cause another atrial depolarization. Continuing repetition of the cycle around this re-entry loop results in a reciprocal rhythm. Interruption of this rhythm can be achieved by a properly timed premature best interferring with the impulse propagation in the re-entry loop. Circulation of the re-entry impulse will then stop because refractory tissue will be encountered in the re-entry pathway. If the pacemaker can apply the properly timed impulse through an electrode having access to the re-entry circuit, refractoryness of tissue can be created ahead of the circulating impulse, thus breaking the tachycardia repetitive cycle. Usually there is only a short time interval in each cycle, a narrow time window, during which a premature beat is effective in terminating the tachycardia.
One prior art technique is to provide a pacemaker that delivers a burst of high frequency pulses during a predetermined time interval following a beat to blanket the critical time window and thus to provide the necessary pulse to break the tachycardia cycle. Although this technique is successful in many cases, it unfortunately cannot be used with some classes of patients to whom the pulse burst might be harmful.
Other prior art techniques have used a demand type pacemaker which is switched into a fixed rate, or asynchronous mode, to treat the tachycardia. In an asynchronous mode, the varying phase relationship between the stimulating pulses and the tachycardia beats is such that within a short period of time an asynchronous pulse will occur at the critical time to interrupt the tachycardia, following which normal demand pacing can be resumed.
One method of treatment of tachycardias by asynchronous pacing involves the use of an implanted demand type pacemaker which can be switched to asynchronous operation by placing an external magnet over the unit. This technique is obviously of limited utility since it depends upon the patient to recognize the tachycardia and initiate the treatment, and there is a risk that the patient may be incapacitated when the tachycardia occurs. Another technique involves the use of an implanted demand pacemaker having circuits to sense a tachycardia rhythm and then automatically revert to fixed rate pacing for the duration of the tachycardia. While this technique has proved useful, potential problems still exist in several areas. Selection of the site within the heart for placement of the electrodes may present a problem in some cases, and unwanted reversion to fixed rate pacing upon occurrence of a single, or a small number of premature beats can also present problems.